Platelets have a crucial role in vasculature maintenance of tumors besides their well-known role in hemostasis. Tumors grow and become metastatic at the expense of platelet activation to build their own unstable vascularization. Experiments where solid tumors injected to mice were platelet-depleted, reveal the development of hemorrhages specifically at the tumor site. These investigations demonstrated the importance of platelets in the maintenance of tumor vasculature stability. Tumors are well-known sites of angiogenesis and inflammation, and platelets are key mediators in both of these events, thus it is important to decipher the cellular crosstalk between platelets and endothelial cells to further develop effective anti-tumor treatments. So far, we know that platelets are important reservoirs of pro- and anti-angiogenic molecules and available anti-cancer treatment has been developed from the manipulation of these molecules in the blood, such as anti-VEGF antibodies. The use of anti-platelet molecules, such as cyclooxygenase (COX2) inhibitor aspirin and the P2Y12 receptor inhibitor ticagrelor, have been shown to provide benefits in cancer patients and in mouse models, respectively. Therefore, the identification of molecules that inhibit platelet aggregation could provide therapeutic potential.
We previously reported the TREM-Like transcript-1 protein (TLT-1) which is a 37 kDa transmembrane protein that is highly expressed in platelets and megakaryocytes. TLT-1 has three reported isoforms in platelets: TLT-1 full length (TLT-1), soluble TLT-1 (sTLT-1) and TLT-1 splice variant (TLT-1sv). After platelet activation these cells release a soluble extracellular fragment of TLT-1 (sTLT-1) that is detected in the serum but not in plasma of healthy mice or humans. Interestingly, TLT-1 is the platelet's fourth most abundant molecule in the platelet releasate. Further studies on this protein revealed that it has a role in hemostasis, as TLT-1 knockout mice showed predisposition to hemorrhage associated with a localized inflammatory lesion and had significantly longer tail bleeding times than normal mice. In addition, blockade of TLT-1 with a single chain antibody prevented thrombin-induced platelet aggregation. Morales et al (2009) showed how recombinant sTLT-1 enhanced platelet-endothelial cell adherence, suggesting that sTLT-1 could mediate hemostasis by enhancing actin polymerization in platelets and resulting in increased platelet aggregation and adherence to the endothelium. Interestingly, elevated levels of sTLT-1 are associated with both a role for TLT-1 in disease and the presence of disseminated intravascular coagulation (DIC). Cancer is one of a handful of diseases that are associated with disseminated intravascular coagulation (DIC). We have subsequently demonstrated in a nude mouse model, using sTLT-1 as a measure of DIC (because it correlates better than D-Dimers), that sTLT-1 levels are higher in mice with larger tumors than those with smaller tumors, suggesting a role for TLT-1/sTLT-1 in cancer progression.
Considering the therapeutic relevance of the released platelet-factors in tumor vasculature, the present invention demonstrates the signaling mechanisms of sTLT-1 release in platelets and a novel direct crosstalk between sTLT-1 and endothelial cells that leads to an increase in endothelial cell actin polymerization, the generation of pro-angiogenic features, and proangiogenic cytokine release in endothelial cells. Accordingly, the present invention proposes a new potential role of TLT-1 in the mediation of angiogenesis and a specific ant1-TLT-1 antibody that interacts with a CDR3 loop of TLT-1.